Ultra low residual reflection, low stress lens coating

ABSTRACT

A method is provided for coating optical lenses and other optical articles with anti-reflection (AR) coatings. The lenses have low reflectivity, provide a substantially white light reflection and have a low stress AR coating and are ideally suited for optical lenses made using a molding procedure which provides a low stress lens substrate. In one aspect the method uses special coating compositions with one being a high index of refraction composition and the other being a low index of refraction composition. In another aspect a method is also disclosed using an optical monitor in conjunction with a conventional vapor deposition apparatus whereby an optical reference lens is used and a particular light frequency of reflected light is measured and this measurement is then used to determine when the desired optical coating is achieved. In a still further aspect the method also preferably calculates the optical thickness of each layer using a specific ratio of blue to green to red colors in the reflected light. The stress of the AR coating is also controlled by adjusting the optical thickness for each layer, if necessary, to minimize the difference in the tensile stresses and compressive stresses between low index/high index layers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to low stress, low residual reflectionmulti-layer anti-reflection coatings for optical lenses and, inparticular, to a composition for forming a high refractive indexanti-reflection coating and a composition for forming a low refractiveindex anti-reflection coating, methods for making optical lensespreferably using the composition including using a conventional vacuumdeposition chamber employing an optical monitor to control the opticalproperties of the anti-reflection coating.

2. Description of Related Art

It is well known in the optical arts that a reflection of light offglass and other surfaces is undesirable or creates visual sightdiscomfort. The reflected light makes a user feel dizzy or causes animage to be blurred, among other such undesirable effects. For opticallenses this is of particular concern and compositions and methods havebeen developed for reducing the reflection of light off the optical lenssurface.

A considerable number of anti-reflection (AR) coatings have beensuggested in the prior art for a primary design purpose of ensuring thatthe residual reflection will be held to a relatively small value overthe entire range of the visual spectrum. Single or double layer coatingshave provided significant improvement but the residual reflections arestill more than desired and to improve the AR properties, the prior arthas resorted to AR coatings having three or more layers.

The optical thickness of each deposited AR layer is typically controlledto optimize or maximize the AR effect and as well known the opticalthickness of a layer is the product of the real (geometrical) thicknessand the refractive index of the respective layer. The optical thicknessis generally described in fractions of a wavelength of a designatedreference light ray for which the coating is to be used. Frequently, thedesign wavelength will be about 510 nanometers (nm) to 550 nm. Theoptical thickness of respective AR layers may be defined by thefollowing general formula where N is the refractive index, d is thegeometrical thickness of the layer and λ is the reference wavelength:N_(a)d_(a)=xλwherein x is a number, typically a fraction, indicating the fraction ofthe wavelength and a is an integer representing the layer coated withthe lowest number being closer to the eyeglass lens. Typically x will be0.25 which represents a quarter wavelength optical thickness.

As well known in the art today, the optical thickness of the individuallayers can be adjusted to obtain the same results on substrates ofdifferent refractive indices.

In the formation of each AR layer, the deposited layer exhibits amaximum value of interference for every one fourth of the wavelength oflight for measurement of the thickness, i.e., λ/4. Thus, the thicknessof an optical AR layer is conventionally controlled during the formationthereof by utilizing this phenomenon with the optical thickness beingmultiples of 0.25.

While the following description will be directed to polycarbonate lensfor convenience, it will be understood to those skilled in the art thatthe invention applies to other lens materials such as polyurethane,acrylic glass, CR-39, etc. Stress in a polycarbonate lens causesbirefringence and optical distortion. While not visible under normalcircumstances it is evident when polycarbonate is placed between twopolarized filaments and this is one of the reasons that polycarbonatelenses are optically inferior to lenses such as glass, CR 39 and othersuch materials. The new polycarbonate lens developed by Optima, tradename Resolution®, is free of this stress and birefringence and thus thecurrent processing to provide AR coatings and its inherent stress nowbecomes more of a concern to makers of such lenses.

In addition, the current state of AR coatings have a residual greenreflection which varies between 0.75% and 1.5% residual reflection. Thisgreen color is cosmetically unpleasant and acts as a green filter whichdecreases the amount of green light the human eye perceives. A lowerresidual reflection with no filtering effect is much more desirable bothin the performance of the coating and in its cosmetic appearance. It ispreferred that only white light be reflected.

The current design and production of AR coatings are well understood inthe industry today and typically the residual color is left in thedesign to make manufacturing much simpler and cheaper. Currenttechnology uses a Quartz Crystal Monitor to control the physicalthickness of the individual layer required to produce an AR coating. Thecurrent coating standards call for a 4-layer HLHL coating, where Hrepresents a high index dielectric material chosen for its specificrefractive index, and L represents a low index dielectric material alsochosen for its refractive index. Each layer typically consists of anoptical quarter wave of the high or low index material chosen. Low indexmaterials include SiO₂ and MgF₂. High index materials include oxide subgroups of the following materials: Zr, Hf, Ta, Ti, Sb, Y, Ce, and Yb.While not inclusive, these materials are the most widely used today.

Many AR coatings being produced today also include an adhesion layer, abuffer layer, an abrasion resistance layer, and a hydrophobic outerlayer. These layers are used to enhance the performance of the coatingfrom a consumer standpoint but have very little effect on the opticalqualities of the AR coating.

Another concern in the making of AR coatings is that the high index andlow index material induce both compressive as well as tensile stress inthe AR coating film. The current art of anti-reflection (AR) coatings,however, does not take into account the amount of stress inherent in thecoating itself. This is because the current lens produced on the markettoday such as the polycarbonate lens has so much stress already that theadditional amount of stress caused by the AR coating is not consideredimportant. This is one of the reasons that current production techniquestry to limit the number of layers used. In general, a low index materialsuch as silica produces a tensile stress which is about 5 times thecompressive stress produced by a high index material. If the coatingbecomes too thick with additional layers, the differences in stresscaused by the low index material and high index material can cause theAR film to separate and come off the lens and also cause adverse opticaleffects.

Another reason current technology limits the number of layers is thatthe quartz crystal monitor is only capable of measuring the physicalthickness of the applied materials. An AR coating however is designedaround optical qualities, which are very dependent on the refractiveindexes of the materials being used. These indexes will shift as coatingconditions such as available O2, coating rate and deposition temperaturechange. The green reflectance left in the coating does an excellent jobof hiding these imperfections during normal production and the high peakreflectance in the very broad green visible spectrum can shift duringproduction and be unnoticeable to all except a well trainedprofessional.

In order to form an AR coating with no residual color, i.e. white, and alow overall residual reflection, the manufacturer must typically addseveral additional AR layers. The added thickness created by theselayers causes an increase in stress and possible AR coating delaminationand these competing problems must be addressed by the lens manufacturer.

Bearing in mind the problems and deficiencies of the prior art, it istherefore an object of the present invention to provide a compositionfor making a high index of refraction AR coating on an optical lens orother optical article.

It is another object of the present invention to provide a compositionfor making a low index of refraction AR coating on an optical lens orother optical article.

It is yet another object of the present invention to provide a methodfor making optical lenses or other optical articles having an AR coatingusing the above compositions.

It is still yet another object of the present invention to provide amethod for making optical lenses having an AR coating using an opticalmonitor to provide a desired AR optical coating on the optical lens orother optical articles.

A further object of the present invention is to provide a method forcoating optical lenses and other optical articles with an AR coatingwhich has low residual reflection, the reflective light is essentiallywhite light and the AR coating has low stress.

A further object of the present invention is to provide optical lensesand other optical articles made using the methods of the invention.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

SUMMARY OF THE INVENTION

The above and other objects and advantages, which will be apparent toone of skill in the art, are achieved in the present invention which isdirected to, in a first aspect, a composition for making a high index ofrefraction AR coating on an optical lens comprising a mixture of ceriumand titanium oxides wherein the cerium oxide is less than about 25% byweight of the composition.

In a further aspect of the invention, a composition is provided formaking a low index of refraction AR coating on an optical lenscomprising a mixture of silicon and aluminum oxides wherein the aluminumoxide is less than about 10% by weight of the composition.

In still another aspect of the invention a method is provided for makingoptical lenses having an anti-reflection (AR) coating comprising thesteps of:

-   -   supplying one or more optical lens and an optical reference        lens;    -   positioning the lens and the optical reference lens in a vacuum        deposition chamber in the same coating plane, the vacuum        deposition chamber having an optical monitor communicating with        the optical reference lens;    -   providing in the chamber at least one source of a high index of        refraction AR coating composition and at least one low index of        refraction AR coating composition;    -   applying a layer of the high index of refraction composition on        the lens until the desired optical thickness coating is obtained        as determined by the optical monitor;    -   applying a layer of the low index of refraction composition on        the lens until the desired optical thickness coating is obtained        as determined by the optical monitor; and    -   repeating the AR application steps until the desired AR coating        is applied;    -   wherein the optical monitor comprises means for directing an        on/off beam of light into the chamber at the optical reference        lens, measuring the reflected light from the reference lens at a        particular frequency and using this measurement to determine        when the desired optical coating thickness is achieved.

In another aspect of the invention a method is provided for makingoptical lenses having an anti-reflection (AR) coating comprising thesteps of:

-   -   supplying an optical lens;    -   positioning the lens in a vacuum chamber of a vacuum deposition        apparatus;    -   providing in the vacuum chamber a source of at least one high        index of refraction AR composition and at least low index of        refraction AR composition wherein one of the high index        materials comprises a mixture of cerium and titanium oxides and        one of the low index materials comprises SiO₂;    -   applying a layer of the high index material on the lens until        the desired optical thickness coating is applied;    -   applying a layer of the low index material on the lens until the        desired optical thickness coating is applied; and    -   repeating the application steps until the desired        anti-reflection coating is applied.

In another aspect of the invention a method is provided for makingoptical lenses having an anti-reflection (AR) coating comprising thesteps of:

-   -   supplying an optical lens;    -   positioning the lens in a vacuum chamber of a vacuum deposition        apparatus;    -   providing in the vacuum chamber a source of at least one high        index of refraction AR composition and at least low index of        refraction AR composition;    -   applying a layer of the high index material on the lens until        the desired optical thickness coating is applied;    -   applying a layer of the low index material on the lens until the        desired optical thickness coating is applied; and    -   repeating the application steps until the desired        anti-reflection coating is applied;    -   with the proviso that the reflected light off the        anti-reflection coating be controlled so that the ratio of blue        light to green light to red light provides a substantially white        reflected light.

In a further aspect of the invention, the optical thickness of the ARcoating layers are adjusted, if necessary, to minimize the difference intensile stress and compressive stress in the adjacent layers.

In another aspect of the invention an optical lens or other opticalarticle is provided which is made by the above methods.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel and the elementscharacteristic of the invention are set forth with particularity in theappended claims. The figures are for illustration purposes only and arenot drawn to scale. The invention itself, however, both as toorganization and method of operation, may best be understood byreference to the detailed description which follows taken in conjunctionwith the accompanying drawings in which:

FIG. 1 is a schematic illustration of a conventional vacuum chamber usedto deposit coatings on substrates and the optical monitor of theinvention used in conjunction with the vacuum chamber.

FIG. 2 is an illustration of a lens containing an anti-reflectioncoating made using the composition and method of the invention.

FIG. 3 is a graph showing the reflectance (percent) as a function ofwavelength for a conventional anti-reflection coating vs. ananti-reflective coating made using the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In describing the preferred embodiment of the present invention,reference will be made herein to FIGS. 1-3 of the drawings in which likenumerals refer to like features of the invention. Features of theinvention are not necessarily shown to scale in the drawings.

Applicants have invented AR coating compositions in both the low andhigh index ranges, which allow control of the AR coating with regard toresidual reflection and the coating stress produced. This allows thenumber of AR layers used to be increased significantly if necessary toachieve the desired lens. Applicants also use an optical monitor tocontrol the optical thickness and rate of the materials being coated.The optical monitor uses a special test glass, which receives thecoating material at the same time as the lens. By measuring the coatingoptically in-situ, one can automatically correct for any minor changesin refractive index and stop the coating of the layer at the preciseoptical thickness required. This is extremely important as any errorinduced in one layer can cause each subsequent layer to be mismatched.In this respect, the optical monitor besides being optically correct canalso make minor corrections in subsequent layers if required.

The end result of Applicants' invention is a cosmetically pleasingcoating, with low residual unwanted color, low reflectance and lowstress.

Although the present invention has been described with reference to aspecific embodiment, those of skill in the art will recognize thatchanges may be made thereto without departing from the scope and spiritof the invention as set forth in the appended claims. While the ARcoating was developed specifically for polycarbonate lenses, thetechniques described can be used for any lens materials, organic orinorganic, including glass, CR-39, and lenses with indexes ranging from1.40 to >1.90.

Referring now to FIG. 1, a conventional vacuum chamber is showngenerally as 10 for depositing an anti-reflection coating on lenses andincludes an optical monitor shown generally as numeral 30.

Any conventional vacuum coating apparatus can be used and exemplary arethose shown in U.S. Pat. Nos. 3,695,910; 5,026,469; and 5,124,019, whichpatents are hereby incorporated by reference.

The vacuum chamber comprises a chamber 11 having a transparent section18 at the top of the chamber. In the vacuum chamber are positionedcontainers 12 a, 12 b, 12 c and 12 d, which are used to hold coatingmaterials 13 a, 13 b, 13 c and 13 d, respectively. It will beappreciated by those skilled in the art that the number of containersand coating materials will vary depending on the anti-reflectioncoatings desired to be applied to the lens substrate.

An E-gun 14 is shown which is used to provide electrons which aredirected at the various containers to volatilize the material in thecontainer. Depending on the material to be volatilized, the container ismoved into position so that the E-gun electrons are directed at thecontainer and material. The material is vaporized and is spreadthroughout the chamber as shown by the arrows. A substrate holder 15 isshown which is curved (typically a dome) and the volatilized material isapplied evenly on all the substrate surfaces. A distribution shield istypically used to apply the volatilized materials evenly. Foursubstrates are shown as 16 a-16 d. Typically 75-140 substrates arepositioned on a dome. A reference substrate 17 is positioned in thecenter of the substrate holder 15 and, it too will likewise be coated bythe volatilized material at the same rate and with the same compositionas the other substrates 16 on the substrate holder 15. Input 32 is usedtypically for gases such as O₂ which is used to form oxides for some ARlayers.

In operation, the desired container and coating material will be movedin position in the vacuum chamber and the E-gun activated to directelectrons at the container to volatize the coating material. The coatingmaterial will be vaporized and the vapors coat each of the substrates 16held by the substrate holder 15. The reference substrate 17 willlikewise be coated. Such a coating process and vacuum chamber areconventional and well known in the art as shown in the above patents.Vacuum deposition is preferred but other methods such as sputtering maybe used.

During the coating operation, it is preferred to use an optical monitorand a high intensity beam of light 20 is projected from light source 19.The beam of light 20 passes through a light chopper 21 which turns thebeam on and off providing an on/off beam 22. The sequence of the on/offlight is synchronized with the light detector 29 at the end of themonitor. This is important since during the off period of the lightbeam, the light detector 29 will still receive a large amount of ambientlight. Since it is programmed that the light it is receiving during theoff period is noise, it subtracts it from the amount of light itreceives when the light beam is on. This ensures that only the lightwhich it is supposed to be measured, is in fact measured.

The chopped light 22 also passes through a focusing lens 21 a and isthen directed towards a high reflectance mirror 23. The reflectancemirror 23 turns the beam as reflected beam 24 towards the transparentopening 18 in the chamber at the reference substrate 17 which isdisposed inside the chamber. As noted above, the reference substrate 17is located in the same curvilinear plane as the substrates 16 which areto be coated. This ensures that during the actual AR coating process thereference substrate 17 receives the same coating of AR material as eachof the substrates being coated.

When the reflected beam 24 reaches the reference substrate 17, themajority of the light passes through the reference substrate.Approximately 5% of the light from the back surface and 5% of the lightfrom the front surface are reflected. It is preferred that the beam oflight enters the chamber at a small incident angle so that the lightbeam reflected from the front and back surface of the monitor glassreturn at a slightly different angle. This is important because only thelight being reflected from the front surface of the reference substrateis to be measured. This reflected light from the front surface is shownas second reflected beam 25. The beam reflected from the back surface isnot shown.

The 5% of the original light which is being reflected back from thefront surface as beam 25 now passes out of the chamber throughtransparent section 18 and hits a second reflection mirror 26 and isturned towards the light detector 29. Before reaching the detector 29light beam 25 passes through a light filter 27 which is a frequencyspecific filter designed to allow only one frequency of light to passthrough the filter. This specific frequency light is shown as beam 28which beam is then passed into light detector 29.

The method of the invention provides an optical coating which isaccurate at a specific desired light frequency. Because of this, inorder to design and form optical coatings, the AR coating thicknessdesired must be designed through a specific light frequency. The lightfilter 27 is chosen to pass only the light frequency chosen by thedesigner when designing the AR coating. Typically, the frequency is 480to 530 nm.

The specific frequency light continuing into the detector 29 is thenmeasured for the amount of light received and the detector amplifies thelight to a more accurate, readable intensity. Through the use ofhigh-resolution A/D converters and microprocessors, the detector iscapable of detecting light changes as small as 0.01%. The light detector29 sends the light intensity data to the vaporization control system 31which uses the information to determine the optical thickness for eachlayer of material being applied and when to stop the vaporizationprocess for the material to be applied when the desired opticalthickness is coated on the lens. It should be noted that it is becausethe optical monitor 30 is reading the change in optical performanceduring the coating process at the same time as the actual change inoptical performance on the lens surface which makes the monitor soaccurate. The monitor 30 also allows the system to make minorcorrections for shifts in refractive index during the coating process.It should be appreciated that the monitor 30 relies strictly on opticalperformance of the coating and not material physical thickness which iscoated on the substrate surface.

The AR coating stress can also be controlled as discussed above tochange the design AR coating optical thickness to minimize thedifference between the tensile and compressive forces in the layers.Changes in optical thicknesses will generally be in steps of 0.5λ sincethis has no significant effect on the optical properties.

FIG. 2 is a representation of the AR coating of the invention on a lenssubstrate. All coatings begin at the substrate and are coatedsequentially outward, both in design and in the actual manufacture. Thesubstrate shown is a stress-free polycarbonate optical lens. This lenswas made using a patented process as shown in U.S. Pat. No. 6,042,754,assigned to the assignee of the subject invention. Although the processbeing described is for this particular lens, it can also be used on anylens material having refractive indexes of 1.40 through 1.9, or higher,with modifications to the AR layer thicknesses to compensate for thedifferent lens materials. All thickness measurements are in Quarter WaveOptical Thickness (QWOT) (0.25λ). The frequency of the light used todesign the formula and used during the actual production process isbetween 470 nm and 580 nm. The AR coating layers were calculated asdiscussed herein by controlling the ratio of the amount of blue light togreen light to red light in the light reflected off the coated opticallens. Blue was controlled to 37.16%, green to 28.57% and red to 34.27%.It will be appreciated that the calculated optical thicknesses can bevaried somewhat to accommodate manufacturing requirements.

Details of the lens shown in FIG. 2 are as follows:

Substrate 51—Polycarbonate lens with a refractive index of approximately1.59.

Primer 52—A primer is applied to the lens so that the final hard coatwill adhere more readily. Approximate thickness is 0.5 to 1.0 microns.Refractive index of 1.50.

Hard Coat 53—A polysiloxane based thermal cure material with a thicknessof between 3.5 to 5.0 microns. Refractive index of 1.49.

L1 54—A low index material such as SiO₂. The thickness is approximately1.70-1.9 QWOT. The refractive index is approximately 1.45-1.5.

H1 55—A high index material of the invention designed to have lowerstress and increased refractive index. The thickness is approximately0.10-0.25 QWOT. The refractive index is approximately 2.04-2.30.

L2 56—Same material as L1. Thickness is approximately 0.10-0.25 QWOT.

H2 57—Same material as H1. Thickness is approximately 1.00-1.25 QWOT.

L3 58—Same material as L1. Thickness is approximately 0.01-0.1 QWOT.

H3 59—Same material as H1. Thickness is approximately 1.25-1.50 QWOT.

M1 60—A middle index material used to help increase adhesion and improvescratch resistance. The thickness is approximately 0.0°-0.1 QWOT.

L4 61—Same material as L1. Thickness is approximately 1.75-2.00 QWOT.

Hydro 62—A polysiloxane material applied to the outer surface to form asmooth slick surface. It improves the cleanability of the lens. Theapproximate thickness is 0.01-0.25 QWOT. The refractive index isapproximately 1.40-1.50.

The lens was found to have low stress, low reflection and low residualcolor, i.e., the reflected light was essentially white. The final lenshas a curve similar to curve 70 in FIG. 3.

FIG. 3 shows graphically the difference between the AR coating of theinvention and a typical AR coated lens currently available on themarket. This graph only shows the optical superiority, and not thedecreased stress capability of the coating. Curve 70 represents theresidual reflection found on AR coatings being produced for the markettoday and shows peak 70 a which is in the green spectrum and producesthe residual green reflectance of conventional lenses. It should also benoted that minima points 70 b and 70 c represent blue light and redlight reflectance, respectively.

As discussed previously, this is commercially acceptable since it hidesthe fluctuations in coating thickness during the production process. Thepeak reflection 70 a (highest point on curve) can be adjusted to theright or left by moving the entire curve right or left. The result is tochange the green residual color to a more bluer appearance or a yellowerappearance. In addition, an AR coating company can rotate the curve sothat the minimum on the right side of the curve comes up to around 0.75%reflection. The result is that the total amount of residual reflectionrises quite significantly. The other result is that the residual colorhas a definite greenish yellowish appearance.

Curve 71 represents the AR coating of the invention as shown in the lensof FIG. 2. Notice that the total residual reflection is much lower thanthe conventional curve 70. Also notice that the curve extends furtherinto both the infrared and ultraviolet regions of the visible lightspectrum (wider). This is a significant factor since all AR coatings onlenses will have a tendency to change color as the angle of the incidentlight (angle of light hitting the surface) becomes less and less direct.This apparent change in color is caused by the curve shifting to theleft as the angle of incidence increases. The narrower the total curve,the quicker it changes color. It is very noticeable because suddenlygreen becomes yellow, orange or red. Curve 71 has a much broader widthand also has no color. As the angle of incidence increases, the curvewill begin to shift left, but the color will remain unchanged until theangle is extremely steep such as up to 45°.

Applicants' invention is directed in one aspect to modifying theconventional curve shown as numeral 70 to a white light curve such asshown as numeral 71. The white light curve 71 has a combination ofcolors that produces a white light reflection and does not have apredominant green reflection as shown in the conventional curve 70.

Applicants have discovered that adjusting the ratios of blue light,green light and red light to each other in the reflected light from theanti-reflection coating will produce a curve shown as numeral 71 whichproduces a substantially white light. It is known to use computersoftware to calculate thin film thicknesses for optical lenses byspecifying certain optical parameters which the computer software willuse to calculate and provide thin film thicknesses for the AR coating.Merely specifying, for example, that the blue, green and red levels arethe same will not produce a white light but will provide a curve such ascurve 70 which has a green peak and a residual green reflection.

It is an important feature of Applicants' invention that the ratio ofblue peak, green peak and red peak in the reflected light be controlledto provide a white light reflection. The three colors are controlledwithin a particular ratio to produce a white light reflection. Ingeneral, in color peak percent, the blue peak will range from about 34to 40%, preferably 36-38%, e.g., 37%, the green about 24 to 32%,preferably 26-30%, e.g., 29%, and the red about 30 to 38%, preferably32-36%, e.g., 34%. When these ratios are supplied to the computersoftware along with other optical properties such as the refractiveindices of the materials being used and a table of the refractiveindices over a range of optical thicknesses, the software will calculatethe AR layers needed to produce the specified blue, green and red peaks.A typical computer software program is called “Essential MacLeod”,Optical Coating Design Program, Copyright Thin Film Center, Inc.1995-2003, Version V 8.6, which program is distributed by the Thin FilmCenter, Inc. Other similar known software programs can be used tocalculate thin film thicknesses which meet the above ratios. It shouldalso be appreciated that the optical thicknesses needed to meet theabove ratios can be calculated manually as is known in the art. Atypical calculation method is shown in U.S. Pat. No. 4,609,267, whichpatent is hereby incorporated by reference, but other known methods forcalculating optical thicknesses can be used.

In another aspect of the invention it is important that the AR coatinghave low stress since high stress causes optical distortion and the ARcoating may delaminate. It has been found that the high index materialand the low index material have different stresses when formed as thinfilms and it is a feature of the invention to minimize the difference inthe stresses in the layers to produce an AR coating having low stress.

For example, it has been found that silicon dioxide which is a typicallow index material provides a tensile stress when coated. On the otherhand, high index materials typically provide a compressive stress whencoated. It has been found, however, that the compressive stress isusually less than the tensile stress of the low index material.Accordingly, this provides a difference in tensile and compressivestresses between layers and could lead to delamination and opticaldistortion.

It is thus an important feature of Applicants' invention to adjust, ifnecessary, each adjacent layer of the optical coating to balance thetensile stress and compressive stress. This is accomplished by firstcalculating the optical thicknesses for the various layers as describedabove specifying the desired reflective peaks (ratios) for blue, greenand red light. Once the optical thickness and number of AR layers aredetermined by the computer calculations, the optical thickness of eachlayer may be modified in 0.5λ steps to balance (equilibrate) the stressbetween layers. For example, if a low index layer has an opticalthickness of 0.25λ and provides a tensile stress of 5 and the adjacenthigh index layer having also a 0.25λ optical thickness produces acompressive stress of only 1, the optical thickness of the high indexlayer is preferably increased to increase the compressive force tobalance or minimize the higher tensile stress of the preceding low indexlayer. In this example, the optical thickness of the high index layercould be adjusted to 0.75λ or even 1.25λ to increase the compressivestress to be closer to the low index layer tensile stress. Increasingthe optical thickness of one layer vs. the adjacent layer will have nosignificant effect on the white light reflection of the coated lensbecause the optical thickness will be typically increased in 0.5λ steps.

While the present invention has been particularly described, inconjunction with a specific preferred embodiment, it is evident thatmany alternatives, modifications and variations will be apparent tothose skilled in the art in light of the foregoing description. It istherefore contemplated that the appended claims will embrace any suchalternatives, modifications and variations as falling within the truescope and spirit of the present invention.

1. A composition for making a high index anti-reflection coating on anoptical lens comprising a mixture of cerium and titanium oxides whereinthe cerium oxide is less than about 25% by weight of the composition. 2.A composition for making a low index of refraction AR coating on anoptical lens comprising a mixture of silicon and aluminum oxides whereinthe aluminum oxide is less than about 100% by weight of the composition.3. A method for making optical lenses having an anti-reflection (AR)coating comprising the steps of: supplying one or more optical lens andan optical reference lens; positioning the lens and the opticalreference lens in a vacuum deposition chamber in the same coating plane,the vacuum deposition chamber having an optical monitor communicatingwith the optical reference lens; providing in the chamber at least onesource of a high index of refraction AR coating composition and at leastone low index of refraction AR coating composition; applying a layer ofthe high index of refraction composition on the lens until the desiredoptical thickness coating is obtained as determined by the opticalmonitor; applying a layer of the low index of refraction composition onthe lens until the desired optical thickness coating is obtained asdetermined by the optical monitor; and repeating the AR applicationsteps until the desired AR coating is applied; wherein the opticalmonitor comprises means for directing an on/off beam of light into thechamber at the optical reference lens, measuring the reflected lightfrom the reference lens at a particular frequency and using thismeasurement to determine when the desired optical coating thickness isachieved.
 4. A method for making optical lenses having ananti-reflection (AR) coating comprising the steps of: supplying anoptical lens; positioning the lens in a vacuum chamber of a vacuumdeposition apparatus; providing in the vacuum chamber a source of atleast one high index of refraction AR composition and at least low indexof refraction AR composition wherein one of the high index materialscomprises a mixture of cerium and titanium oxides and one of the lowindex materials comprises SiO₂; applying a layer of the high indexmaterial on the lens until the desired optical thickness coating isapplied; applying a layer of the low index material on the lens untilthe desired optical thickness coating is applied; and repeating theapplication steps until the desired anti-reflection coating is applied.5. The method of claim 4 wherein the cerium oxide is less than about 25%by weight of the composition.
 6. The method of claim 5 wherein thealuminum oxide is less than about 10% by weight of the composition.7.-10. (canceled)
 11. An optical article made by the method of claim 3.12. An optical article made by the method of claim
 4. 13. An opticalarticle made by the method of claim
 5. 14. An optical article made bythe method of claim
 6. 15. An optical article made by the method ofclaim
 7. 16. An optical article made by the method of claim
 8. 17. Anoptical article made by the method of claim
 9. 18. An optical articlemade by the method of claim 10.